The invention concerns novel associative amphoteric polymers for increasing the viscosity of aqueous solutions, as well as aqueous compositions containing said polymers.
By definition, we say that a polymer is polymer viscosifying when it increases the viscosity of the solutions in which it is dissolved.
Among the viscosifying polymers for aqueous solutions included in the state of the art, synthetic polymers such as poly(meth)acrylamides, which may be partially hydrolysed, and poly(meth)acrylates and their copolymers are particularly well known. These polymers develop viscosity due to their molar mass and to interchain ionic repulsions. The mechanism causing viscosity is related to an increase in hydrodynamic volume and interchain repulsions.
However, in the presence of electrolytes, surfactants or a high temperature of use, these polymers do not develop good thickening properties, leading to a sharp decrease in their viscosifying power.
To improve the performances of the aforementioned synthetic polymers, they have been modified by adding hydrophobic groups. The polymers thus obtained (the minority of which have apolar unit cells and the majority of which have hydrophilic unit cells) are called “associative”. They require high molecular mass and are characterised in that, when they are placed in solutions, their hydrophobic groups join together to limit interactions with water. The drop in viscosity related to the presence of electrolytes, surfactants or high temperatures, source of a drop in hydrodynamic volume, is compensated for by interchain attractive interactions.
The vast majority of the most common acrylic associative polymers come in the form of an emulsion (in water or reverse or aqueous dispersion). It should be pointed out that reverse emulsion polymerisation is a preferred method well known to those skilled in the art. It notably provides acrylic polymers with high molecular weight and may contain a reasonably high concentration of polymers while remaining fluid. Moreover, the large specific surface of the dispersed phase of the polymer makes solubilisation or swelling of the polymer very rapid.
The hydrophobic monomers used in preparing these polymers may be of 2 types: non-ionic and/or ionic.
Among the non-ionic hydrophobic monomers, we can mention:                industrially available (meth)acrylic acid esters with an alkyl chain or ethoxylated,        and acrylamide derivatives with an alkyl or dialkyl chain requiring, for co-polymerisation, difficult implementation not yet having undergone any industrial development.        
At the same time, the use of hydrophobic ionic monomers has also been described. This mainly concerns:                cationic allyl derivatives, which have very poor reactivity leading to derivative compounds which are harmful to the desired applications as they form insoluble chains.        or hydrophobic ionic acryloyl derivatives (anionic or cationic), these being mentioned as potential monomers but very rarely exemplified and never described in powder form.        
During research on the preparation of new amphoteric polymers used to increase the viscosity of aqueous compositions, whatever their salinity, their surfactant composition or their temperature, the applicant took a closer look at associative amphoteric polymers.
The previous state of the art shows:                U.S. Pat. No. 5,071,934 concerning the preparation of associative cationic polymers which can be interpreted as amphoteric in extreme cases, obtained in an emulsion. No examples are given concerning polymerisation for obtaining amphoteric polymers: the examples deal with copolymerisation in emulsions of acrylamide, methacrylamido-propyl trimethyl-ammonium chloride (MAPTAC) and dimethyl aminopropylamine (DMAPA) quaternised with an alkyl chain with 8 to 14 carbons.        Patent WO 01/40318 which protects a new method of synthesis in a liquid solution of hydrophobic polymers which may be amphoteric but is not exemplified.        Exxon patent U.S. Pat. No. 4,650,848 which describes non-charged associative hydrophobic AMD/AMD terpolymers/betaine: the amphoteric aspect of these polymers is introduced by a zwitterionic monomer which alone carries both charges in exactly equal quantities.        Calgon patent WO 01/06999 concerning low molecular weight (less than 10,000 g·mol−1) hydrophobic amphoteric polymers, whose apolar groups are carried by an anionic and/or non-ionic monomer. Given their low molecular weight, these polymers have no viscosifying effect.        
None of aforementioned products, however, has to date undergone industrial development due to their inherent inconveniences: lack of viscosity, stability over time, temperature stability, and/or problems related to their preparation or implementation.